Wednesday, July 19, 2006

A trip to China, part 4: traffic (or when local optimization doesn't produce global optimization)

Luckily, I keep independent copies of almost everything I do. Here is the lost fourth installment of my China travel diary.

Riding in a car in Guangzhou is both a frightening and a hopeful experience. I admit there are times when I have had to close my eyes (when riding as a passenger). However, this is still a noticable improvement from even four years ago. Then, the attitude among drivers towards lane markings was probably best summarized as, "What's a lane?" Now, if there's not much traffic, drivers will actually stay in a lane much of the time. When there's not much traffic.

When there is, it's every driver for themselves. Folks will squeeze their cars in between existing lines of moving cars, turning two-lane roads into three or four lane ones. They change lanes regardless of the presence or absence of neighboring cars, trucks, or buses. The make right turns right into moving traffic, and left turns into oncoming traffic, slowly working their way right, dodging oncoming cars until they reach the right side of the road. At first glance, it's amazing that there aren't more accidents. The you realize two things:

  1. accidents are often caused by drivers doing unexpected things and people expect the above behavior, and
  2. nobody is driving very fast -- speeds rarely exceed 35kph.
It's this second thing that becomes most interesting after a while. People are furiously jockeying for every little gap that they can wedge their cars into, but traffic doesn't move very fast even under moderate crowding. It brings to mind laissez-faire capitalism: that a large number of small decisions made by people will result in overall efficiency. In other words, that decisions made that, taken by themselves to be as optimal as possible, will result in global optimization.

Sometimes this works: witness greedy algorithms. But, as Guangzhou traffic shows, this is not always the case. As American and other drivers have learned, sometimes it's better to stay in your lane, rather than cut someone off in a neighboring one. The result is smoother flowing traffic and faster trips for everyone.

Meanwhile, I ponder problems associated with traffic simulation to help keep my mind off the two buses to our right and left that are both merging into our lane...

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